1. Technical Field
The present invention relates to data communications in general, and in particular to data communications utilizing direct access arrangement circuits. Still more particularly, the present invention relates to an apparatus for reducing modem command and status latency on a digital audio interface within a direct access arrangement circuit.
2. Description of Related Art
A direct access arrangement (DAA) circuit is commonly utilized to connect an electrical system to a telephone network. A DAA circuit can provide a communication channel between a host computer system and a tip-and-ring telephone line. A DAA circuit typically includes a system-side circuit and a line-side circuit. The system-side circuit communicates with a host computer system via a communication interface. The communication interface may provide data and control information between the host computer system and the DAA circuitry. In order to allow data communications across a tip-and-ring telephone line, the host computer system may include dedicated modem hardware to provide the necessary modem data processing. Alternatively, a software modem can be implemented within the host computer system by utilizing hardware resources within the host computer system in conjunction with modem software that is being executed on the host computer system.
Regardless of the type of modem being implemented, the DAA circuit must be configured in a manner such that it is compatible with the serial communication interface standard utilized by the host computer system in order for the DAA to take advantage of the serial communication interface of the host computer system. Current serial communication interface standards include the Audio Codec '97 (AC '97) standard and the HD Audio standard.
The AC '97 standard, which is sponsored by the Intel Corporation, provides a uniform interface for computer system audio applications. The current AC '97 standard is enumerated under the AC '97 Component Specification, revision 2.1 (May 22, 1998). The AC-97 Component Specification provides the details for a controller residing in a computer system to communicate with a variety of telecommunication devices, the pertinent of which is incorporated by reference herein. Some of those telecommunication devices, such as modems, are capable of using tip-and-ring telephone lines to communicate with other devices external to the computer system.
The HD Audio standard, which is also sponsored by the Intel Corporation, is an upgrade of the AC '97 standard. The current HD Audio standard is enumerated under the HD Audio Specification, revision 1.0 (2004), the pertinent of which is incorporated by reference herein. Similar to the AC-97 specification, the HD Audio Specification maintains a five-wire audio interface. However, the HD audio interface is based upon a packet-based protocol rather than a time-division multiplex-based protocol.
Any direct access to the hardware registers, such as control and status registers, of a DAA is generally not allowed during software modem communications with the DAA. The software modem must access the hardware registers of the DAA through a system software stack. However, when accessing the system software stack, it is the software, and not the hardware, that determines the fair use of a serial communication interface within a host computer system, which may lead to indeterminate latency for modem command and status communications between the software modem and the DAA. In addition, the system software stack may be responsible for handling multiple devices, including other DAAs and audio codecs, which will further increase the indeterminate latency for modem command and status communications.
In order to meet the various telecommunication standards and to react to harsh telephone line conditions in a timely manner (to prevent equipment damage), minimum response times with high predictability are required for modems that operate on tip-and-ring telephone lines. Thus, indeterminate latency presents major problems to software modems. For example, excessive command latency can cause problems with pulse dialing, caller identification (ID), off-hook/on-hook transitions and enabling overload/over-current protection. Excessive status latency can cause problems with detecting overload conditions, over-current conditions and ring detection.
Consequently, it would be desirable to provide a method and apparatus for reducing modem command and status latency on a serial communication interface within a DAA circuit.